# std::bad_variant_access

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< cpp‎ | utility‎ | variant

Defined in header `<variant>` |  |
class bad_variant_access : public std::exception |  | (since C++17)

`std::bad_variant_access` is the type of the exception thrown in the following situations:

- `std::get(std::variant)` called with an index or type that does not match the currently active alternative.
- `std::visit` called to visit a variant that is `valueless_by_exception`.

`std::variant::visit` called to visit a variant that is `valueless_by_exception`. | (since C++26)

### Member functions

(constructor) | constructs a new `bad_variant_access` object (public member function)
operator= | replaces the `bad_variant_access` object (public member function)
what | returns the explanatory string (public member function)

## std::bad_variant_access::bad_variant_access

bad_variant_access() noexcept; | (1) | (since C++17)
bad_variant_access( const bad_variant_access& other ) noexcept; | (2) | (since C++17)

Constructs a new `bad_variant_access` object with an implementation-defined null-terminated byte string which is accessible through `what()`.

1) Default constructor.
2) Copy constructor. If *this and other both have dynamic type `std::bad_variant_access` then std::strcmp(what(), other.what()) == 0.

### Parameters

other | - | another exception object to copy

## std::bad_variant_access::operator=

bad_variant_access& operator=( const bad_variant_access& other ) noexcept; |  | (since C++17)

Assigns the contents with those of other.If *this and other both have dynamic type `std::bad_variant_access` then std::strcmp(what(), other.what()) == 0 after assignment.

### Parameters

other | - | another exception object to assign with

### Return value

*this

## std::bad_variant_access::what

virtual const char* what() const noexcept; |  | (since C++17)

Returns the explanatory string.

### Return value

Pointer to an implementation-defined null-terminated string with explanatory information. The string is suitable for conversion and display as a std::wstring. The pointer is guaranteed to be valid at least until the exception object from which it is obtained is destroyed, or until a non-const member function (e.g. copy assignment operator) on the exception object is called.

The returned string is encoded with the ordinary literal encoding during constant evaluation. | (since C++26)

### Notes

Implementations are allowed but not required to override `what()`.

## Inherited from std::exception

### Member functions

(destructor) [virtual] | destroys the exception object (virtual public member function of `std::exception`)
what [virtual] | returns an explanatory string (virtual public member function of `std::exception`)

### Example

Run this code

    #include <iostream>
    #include <variant>

    int main()
    {
        std::variant<int, float> v;
        v = 12;
        try
        {
            std::get<float>(v);
        }
        catch (const std::bad_variant_access& e)
        {
            std::cout << e.what() << '\n';
        }
    }

Possible output:

    bad_variant_access

### See also

get(std::variant) (C++17) | reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)
visit (C++17) | calls the provided functor with the arguments held by one or more `variant`s (function template)
visit (C++26) | calls the provided functor with the argument held by the `variant` (public member function)

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